Fe₃ Cluster Anchored on the C₂N Monolayer for Efficient Electrochemical Nitrogen Fixation

Abstract

Under the current double challenge of energy and the environment, an effective nitrogen reduction reaction (NRR) has become a very urgent need. However, the largest production of ammonia gas today is carried out by the Haber–Bosch process, which has many disadvantages, among which energy consumption and air pollution are typical. As the best alternative procedure, electrochemistry has received extensive attention. In this paper, a catalyst loaded with Fe₃ clusters on the two-dimensional material C₂N (Fe₃@C₂N) is proposed to achieve effective electrochemical NRR, and our first-principles calculations reveal that the stable Fe₃@C₂N exhibits excellent catalytic performance for electrochemical nitrogen fixation with a limiting potential of 0.57 eV, while also suppressing the major competing hydrogen evolution reaction. Our findings will open a new door for the development of non-precious single-cluster catalysts for effective nitrogen reduction reactions.

1. Introduction

Ammonia (NH₃) is an important chemical in agriculture and industry [1], and the direct reduction of nitrogen to ammonia is still considered to be one of the most important and challenging chemical transformations [2]. Dinitrogen (N₂), as the component with the largest volume fraction (78.08%) in the Earth’s atmosphere, is the main source of nitrogen [3]. Therefore, ammonia can be directly synthesized from the Earth’s abundant nitrogen resources. Usually, the Haber–Bosch process (N₂ + 3H₂ = 2NH₃) is used to synthesize ammonia on large scales. In this process, iron and ruthenium-based metal catalysts are used to convert atmospheric nitrogen (N₂) into NH₃ by reacting with hydrogen (H₂) [4,5]; the adsorbed N₂ molecule is firstly dissociated on specific active sites of the catalysts [6,7,8], and then the dissociated N species are hydrogenated, which is known as the dissociation mechanism [9]. However, extreme reaction conditions are usually required, such as high pressure (~100 bar) and high temperature (~700 K) [10]; as the reaction proceeds, a large amount of carbon dioxide is emitted, and the energy consumed each year is huge. On the other hand, the conditions of high temperature and pressure may not be necessary because the reaction is actually exothermic [10]. For this reason, it is of significance to search for a green and cost-effective method for the production of ammonia [11].

With continuous efforts, scientists have found that the electrochemical nitrogen reduction reaction (NRR, N₂ + 6H⁺ + 6e⁻ = 2NH₃) [12] under environmental conditions is extremely attractive because of its greatly reduced energy input and good environmental compatibility. Here, the electrocatalyst is the core component for reducing the reaction limit potential and increasing the reaction speed and the selectivity of NH₃ [13]. For NRR, one of the key challenges is the activation of nitrogen, which is due to the highly stable N≡N bond [14]. The capability of N₂ adsorption strongly depends on the electronic structure of the elements in the catalysts, since only the elements possess unoccupied d orbitals; d orbitals with an appropriate symmetry can accept the electrons of N₂, and the occupied d orbitals of these elements can back donate to the π orbitals of N₂, thus weakening the N≡N bond [15]. Suitable and active catalysts can break the N≡N bond, and hence, the single nitrogen atoms react with hydrogen to form ammonia. In recent years, not only single-atom catalysts (SACs) [12,13,14,15,16,17,18,19,20,21] have been emerging as a novel strategy for designing effective electrocatalysts for NRR: the catalytic performance of double atomic catalysts (DACs) [21,22,23,24] and triple atomic catalysts (TACs) [25,26] for NRR has also been explored both experimentally and theoretically.

Many transition-metal (TM) species, such as Fe, Ru, Co and Mo-based complexes, have been used for nitrogen fixation, because the occupied d orbitals of these metals can donate electrons to the empty π*-orbital of N₂ and accept electrons from its σ-orbital, thereby enhancing the adsorption of N₂ [23]. Among the transition metals, Mo and Fe were considered to be the most promising metal species because, based on density functional theory (DFT) calculations, Mo and Fe were located at the top of the volcano map [27], and Fe has received the most attention because of its robustness and low cost [16]. For example, in 2018, Li’s group proposed that Fe₃@Al₂O₃ as a catalyst for NRR has superior catalytic performance [25], and quite recently, Jiang and co-workers’ DFT studies showed that the Fe₃ cluster loaded on a heterostructure of graphdiyne and graphene (GDY/Gra) also had excellent NRR catalytic activity [28]. Thus, we conceived that the Fe₃ clusters should have capable catalytic performance when supported on a suitable substrate, such as the C₂N monolayer. The C₂N monolayer was first fabricated in 2015 [22] and has a unique porous structure originating from a 2D graphene layer, which provides an ideal support for metal atoms as the active center and has a high surface-to-volume ratio property that ensures sufficient exposure of TM atoms to interact with reactant molecules [29]. Thus, we explored the stability of Fe₃ clusters anchored on the C₂N monolayer (Fe₃@C₂N) and the electrocatalysis of NRR by means of density functional theory (DFT) computations. Our calculations show that Fe₃@C₂N is a stable metal and has excellent catalytic performance for the N₂ reduction reaction with the supported Fe₃ cluster serving as the active center.

2. Results and Discussion

2.1. The Geometry, Stability and Electronic Properties of the Fe₃@C₂N

Two-dimensional (2D) C₂N is a porous material similar to graphene, and the hexagonal lattice parameter of the C₂N unit cell was calculated to be 8.32 Å, which matches well with previous studies [29]. The optimized geometric structure of C₂N is given in Figure 1a. In the C₂N monolayer, the C₆ rings are connected by N atoms, resulting in a six-membered nitrogen pore with a para-N distance of 5.51 Å; the pores provide desired sites to anchor metal atoms or clusters. Previous theoretical studies [30,31] showed that adding the second Fe atom to Fe₁@C₂N is energetically favored: the binding energy of the second Fe atom is comparable to that of the first one, and Fe₂@C₂N maintains its original structure throughout a 10 ps molecular dynamics simulation at 800 K [30]. Based on the porous structural feature in the C₂N monolayer and the stable Fe₂@C₂N complex [30,31], we conjectured that adding one more Fe atom to Fe₂@C₂N, i.e., Fe₃@C₂N, may also have good thermodynamic and thermal stability. According to our computations, the lowest-energy supported Fe₃ cluster adopts a buckled triangle structure, with each Fe bonded to two N atoms, and the buckling height of the Fe₃ structure is 1.29 Å, while the C₂N support almost maintains its flat configuration (Figure 1b), similar to the model proposed by Pei et al. [32]. The Fe−Fe and Fe−N bond lengths are in the range of 2.28~2.32 Å (slightly shorter than the 2.48 Å in the bulk phase) and 1.97~2.01 Å, respectively. The average binding energy per Fe atom is −4.12 eV, which is slightly lower than the value of −4.84 eV per Fe atom in the bulk phase at the same level of theory, indicating the good thermodynamic stability of Fe₃@C₂N. Furthermore, a first-principles molecular dynamics (FPMD) simulation in an NVT ensemble with the temperature controlled by the Nosé–Hoover method [33] was performed, and the structure of Fe₃@C₂N was well kept through a 10 ps FPMD with a time step of 0.5 fs at 800 K (Figure S1). Thus, Fe₃@C₂N is extraordinarily stable.

Good electrical conductivity is required for fast charge transfer during efficient electrocatalytic processes. Compared to the semiconducting C₂N monolayer, whose band gap was calculated to be 1.68 eV (Figure 1c), in line with a previous report [29], Fe₃@C₂N is a ferromagnetic (each Fe atom carries the magnetic moment of ~3 μ_B, Figure S2) metal (Figure 1d), and the metallic feature originates from the states that have crossed the Fermi level, which is dominatingly attributed to the Fe-d orbitals (Figure S3). The charge transfer between the Fe₃ cluster and C₂N is 2.12 |e|, indicating the high activity of the Fe₃ cluster.

2.2. N₂ Adsorption on Fe₃@C₂N

Previous investigations proposed the following criteria for an efficient electrocatalyst for NRR: (1) the catalyst can facilitate the chemisorption of N₂ molecules to sufficiently activate the inert N≡N triple bonds, and (2) the catalyst can selectively stabilize N₂H* and (3) destabilize NH₂* species to lower the limiting potential [12]. In the electrochemical synthesis of ammonia, the adsorption of nitrogen is considered to be the first step, and the initial nitrogen adsorption configuration plays an important role in subsequent reactions [9]. On pristine C₂N, the adsorption of N₂ is very weak, and the adsorption energy (E_ads) is only −0.01 eV, which indicates that the pristine C₂N monolayer cannot effectively activate nitrogen. Zhao’s theoretical group also found that N₂ adsorption on C and N atoms is rather weak (E_ads < −0.20 eV), or the N₂ molecule is spontaneously trapped by the central TM atoms after structural optimization on TM@C₂N [34]. In our work, we considered two adsorption configurations, namely, end-on and side-on structures of N₂ on the Fe₃ cluster anchored on C₂N (Figure 2), and the adsorption energies of the two configurations are −1.08 and −1.45 eV, respectively. The E_ads values are sufficiently strong to capture and activate N₂, as indicated by the elongated N≡N lengths of 1.16 and 1.26 Å (the isolated N≡N length was calculated to be 1.11 Å) and a charge transfer of −0.57 and −1.14 |e| for the end-on and side-on configurations, respectively. Compared to the maximum vibrational frequency of free N₂ (2420 cm⁻¹), the maximum vibrational mode of the adsorbed N-N in the side-on (end-on) configuration is 1380 (2011) cm⁻¹. The remarkably reduced frequency in the side-on structure suggests an apparent elongation/weakening in the N-N bond upon N₂ being adsorbed on Fe₃@C₂N.

2.3. N₂ Reaction on Fe₃@C₂N

Considering the energetically preferred side-on configuration and the greater charge transfer between N₂ and Fe₃@C₂N, as well as the much longer stretched N≡N length, we selected the N₂ adsorption with the side-on structure as the starting state for NRR.

Previous studies revealed that the N₂ molecule could dissociate directly on the specific iron surface, such as Fe(111) and Fe(211) surfaces [35,36]; we first examined the dissociative mechanism. However, our computations showed that the dissociated NN state on Fe₃@C₂N is not energetically favored, since it is 2.67 eV higher in energy than the N₂ adsorbed state (Figure S4), suggesting a very high activation energy barrier (>2.67 eV) of N₂ dissociation. For comparison, the calculated barrier of N₂ dissociation is as high as 1.89 eV on Fe₃/θ-Al₂O₃(010) [25]. Thus, the dissociative mechanism of NRR over Fe₃@C₂N was not further studied in our work. Quite recently, Wang’s theoretical group proposed a new mechanism for NRR, namely, a surface-hydrogenation mechanism [37], where the surface hydrogenation can drive the N₂ reduction reaction. We also tested the surface-hydrogenation mechanism of NRR on Fe₃@C₂N; nevertheless, N₂ adsorption on the hydrogenated Fe₃ cluster is endothermic at 2.41 eV (Figure S5), indicating an unfavorable pathway for NRR. Therefore, we focused on the associative mechanism in the following experiment.

For the associative mechanism, we compared the enzymatic (* → *N₂ → *NNH → *HNNH → *HNNH₂ → *H₂NNH₂ → *H₃NNH₂ → *NH₂ → *NH₃ → NH₃(g)) and consecutive (*→ *N₂ → *NNH → *NNH₂ → *NNH₃ → *N → *NH → *NH₂ → *NH₃ → NH₃(g)) pathways [38] for NRR (Figure 3a), and the free energy change for each step is illustrated in Figure 3b,c. The difference between the two mechanisms is that in the enzymatic mechanism, the proton–electron pair (H⁺ + e⁻) alternately attacks two N atoms, while in the consecutive mechanism, the proton–electron pair first continuously approaches one N atom to generate the first NH₃ and then attacks the other N atom to generate the second NH₃ [39]. The free energy difference for each elementary step in the enzymatic (consecutive) route is −0.92, 0.28, −0.02, 0.57, −1.82, 0.11, 0.54, 0.44 and 0.01 (−0.92, 0.28, 0.83, −1.51, 0.28, −0.78, 0.57, 0.44 and 0.01) eV, respectively, and accordingly, the potential limiting step *HNNH → *HNNH₂ (*NNH → *NNH₂) has a maximum free energy change of 0.57 (0.83) eV via the enzymatic (consecutive) mechanism. Thus, the limiting potential (η) of NRR on Fe₃@C₂N is as low as −0.57 V, lower than the η values on the Ru (0001) step surface [40] and Fe₂@C₂N [24] (0.98 eV and 1.23 eV, respectively). Typically, the potential limiting step on most metal surfaces (such as Re, Ru, Rh and Fe) or two-dimensional MBenes is either the first hydrogenation (forming *NNH) or the last protonation (*NH₃ formation or NH₃ desorption) [9,41]. However, on Fe₃@C₂N, the free energy change of *N₂ → *NNH is only 0.28 eV. We also calculated the barriers of the potential limiting steps of the two mechanisms, and the barriers are both 1.11 eV based on the CI-NEB method (Figure S6), lower than the corresponding values (1.24 and 1.31 eV) on Fe₃@θ-Al₂O₃(010) [25]. All of the above results suggest the high activity of Fe₃@C₂N for NRR.

In addition to the catalytic activity, another important aspect for the catalytic performance is the selectivity of NRR [22]. Due to the high content of protons in the acidic solution, the major competitive reaction of NRR is the hydrogen evolution reaction (HER). The adsorption energy of *H on Fe₃@C₂N is −0.94 eV, weaker than E_ads of N₂ (−1.45 eV), and expectedly, the charge transfer between *H and the catalyst is smaller (0.43 |e| vs. 1.14 |e|). By comparing the energies of two *H on Fe₃@C₂N (−1.69 eV) and the formation of H₂ (1.79 eV, illustrated in Figure S7), we found that the reaction energy of HER is as high as 3.45 eV, indicating that Fe₃@C₂N has a high ability to suppress the competing HER during NRR under an acidic environment. The stable and metallic Fe₃@C₂N not only possesses low NRR limiting potential but also exhibits high selectivity against HER, and therefore, the low-cost Fe₃@C₂N is a highly promising electrocatalyst for NRR.

3. Materials and Methods

The spin-polarized density functional theory (DFT) calculations were carried out using the Vienna Ab initio Simulation Package (VASP 5.4.4) [42]. The exchange–correlation functional was described by the Perdew, Burke, and Ernzerhof (PBE) parameterization of the generalized gradient approximation (GGA) [43]. The projector augmented-wave (PAW) potential [44] was employed, and an electron configuration of 3d⁷4s¹ was adopted for Fe. According to our tests, we found that the system is ferromagnetic, with each Fe atom carrying a magnetic moment of ~3 μ_B (Figure S2), which is quite similar to the case of a Fe₃ cluster on the θ-Al₂O₃(010) surface [25]. A cutoff energy of 550 eV was adopted. The van der Waals interactions were described using the empirical correction in the Grimme scheme (DFT-D2) [45], where the dispersion energy is corrected based on the pairwise atomic –C/R⁶ terms. The D2 scheme was widely used in these theoretical studies for the oxygen reduction reaction [29] and NRR [2]. During the structure relation, the maximum force and energy on each atom were less than 0.01 eV/Å and 10⁻⁵ eV, the width of smearing was chosen as 0.2 eV, and k-points were sampled using the 5 × 5 × 1 Monkhorst−Pack mesh [46]. To avoid the interaction between two neighboring surfaces, a vacuum space over 20 Å was used [2,40]. To simulate a C₂N monolayer, a periodic 1 × 1 unit cell (with a lateral dimension of ~8.34 Å) containing 12 carbon atoms and 6 nitrogen atoms was constructed. The barriers of the potential limiting steps were identified by using the climbing image nudged elastic-band (CI-NEB) method [47].

The average binding energy (E_b) of each Fe atom on the C₂N substrate in our work is given by

E_b = (E_Fe3@C2N − E_C2N − 3E_Fe)/3

where E_Fe3@C2N, E_C2N and E_Fe are the energies of the Fe₃@C₂N system, the C₂N monolayer, and an isolated Fe atom, respectively.

The adsorption energy (E_ads) of the single atom or NRR intermediates was determined according to the following equation:

E_ads = E_tot − E_Fe3@C2N − E_adsorbate

where E_tot, E_Fe3@C2N and E_adsorbate represent the total energies of the systems containing the Fe₃@C₂N catalyst and the adsorbate, Fe₃@C₂N, and the adsorbate, respectively.

According to the calculated hydrogen electrode (CHE) model proposed by Nørskov and co-workers [48], the reaction free energy of each basic step ΔG is calculated by

ΔG = ΔE − ΔE_ZPE − TΔS + eU + ΔG_pH

In the above equation, ΔE is the reaction energy difference between the products and reactants of the NRR occurring on the catalyst, which can be directly obtained from DFT computations, and ΔE_ZPE is the change in zero-point energies, which was calculated from the vibrational frequencies [12]. The vibrational frequencies were calculated based on the finite differences method, and only the adsorbed species were included. ΔS is the change in entropy at 298.15 K. Among them, the free energy correction of pH is expressed by ΔG_pH, which is equal to k_BT × ln10 × pH, where k_B is the Boltzmann constant. In this study, it is assumed that pH = 0.

The limiting potential (η) of the entire reduction process is determined by the potential limiting step, which has the most positive ΔG (ΔG_Max), as computed by η = −ΔG_Max/e [49].

The reaction rates of the key reaction steps were estimated based on Nørskov’s model [48], provided that there is no extra barrier: k = k₀exp(−ΔG/k_BT), where ΔG is the free energy change and k₀ is the prefactor. The approximated values are given in Table S3.

4. Conclusions

In summary, we designed a supported catalyst, i.e., Fe₃ clusters anchored on a two-dimensional C₂N monolayer (Fe₃@C₂N), to electrocatalyze the reduction of nitrogen to ammonia. Through our density functional theory computations, we found that Fe₃@C₂N is excellent as an electrocatalyst for NRR owing to the stability at high temperature (800 K), the metallic feature, the absence of precious metal, the low limiting potential (0.57 eV of the enzymatic mechanism) and the high selectivity against the competing side reaction, HER. Such superiority can be ascribed to the partially occupied d orbitals and largely negative charged Fe₃ cluster, which is beneficial in activating the inert N≡N triple bond. Since it was feasible to reach Cu₂@C₂N using CuCl₂ as a metal precursor in a previous theoretical protocol [50], Fe₃@C₂N could be synthesized using FeCl₂ and other proper iron precursors. We hope that our work can inspire more experimental and theoretical studies to further explore the potential of non-precious metal clusters for NRR.